Field of the Invention
The present invention generally relates to apparatus for producing a hard copy from electronic image data by photographic means and, in particular, to applications of such, as in medical imaging, which require a high degree of distinction between shades of gray for purposes of clarity and definition.
Statement of the Prior Art
A great deal of development work has been done in the area of medical imaging which has become an extremely powerful diagnostic tool when coupled with the technologies of CAT scanning, ultrasonic scanning and magnetic resonance imaging. The electronic signals produced by these forms of imaging most readily lend themselves to the use of electronic image data and the production of images through electronic image processing. Such electronic image data is of course easily recorded; However, there is still a significant need for the production of hard copy images for a variety of reasons and purposes. To this end, different photographic printers have been developed. The simplest example of these printers would be the photographing of an image as displayed by a cathode ray tube. More complex printers have also been developed wherein cathode ray tubes and lasers are used to expose a photon sensitive recording media on a scanned, line-by-line basis. Any suitable photosensitive or photon sensitive media may typically be used such as visible light sensitive photographic film, infra-red sensitive photographic film, photo-stimulable phosphor, dry silver paper and so on.
One of the areas of difficulty and therefore a performance criteria for the production of such hard copy, has been the definition of the images so produced. Definition is generally determined by the fineness of the signals or pixels used and the definition between shades in each pixel. The definition between shades is determined by the ability to clearly distinguish on hard copy between close but different signal levels in the image data being used.
In practice, the ability to produce distinctions in shades of gray is limited by the apparatus used to expose the recording media and the recording media itself. The best images thus far, have been produced via laser printing. The definition provided, however, has only come at a high price, due to the need for laser technology and the circuitry necessary to provide quality images therewith. Cathode ray technology is much less expensive than lasers but suffers from several problems in its ability to produce clearly defined shades of gray. These difficulties primarily arise from the difficulty of linearly exposing the recording media in response to the electronic image data. The non-linearities generally arise from each step of the process. The image data, which is typically digital, must first be converted to an analog signal by a digital to analog converter. The next step or source of non-linearities is amplifying the analog voltage, with a video drive amplifier, to a voltage which can be used to modulate the intensity of a cathode ray beam. Still another source of non-linearity arises from cathode ray tubes where the light intensity of the cathode ray phosphor has a non-linear response to the energizing analog intensity voltage. Lastly, inaccuracies are produced by changes in the spot size of the cathode ray phosphor for different electron beam intensities. Although a great deal of development work has been done to reduce these inaccuracies, such as improving the linearity of digital to analog converters, improving the linearity of video drive amplifiers, providing compensation circuitry for the cathode ray phosphor response and improving spot size uniformity, these advances are only available at a cost which reduces the attractiveness of cathode ray printing. Furthermore, the wide variety of parameters which affect cathode ray intensity response can only be optimized at best. They cannot be eliminated.